<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>focus伪类选择器</title>
    <style>
        input:focus {
            background: blue;
          
        }
    </style>
</head>
<body>
   <input type="text"> 
   <input type="text"> 
   <input type="text" > 
</body>
</html>